Superabsorbent crosslinked ampholytic ion pair copolymers

ABSTRACT

The present invention provides crosslinked ampholytic copolymers that are highly absorbent to aqueous electrolyte solutions. The polymers of the present invention comprise polymers formed by the copolymerization of an effective amount of each of the following components to produce a polymer which is highly absorbent to aqueous electrolyte solutions: 
     (a) an ampholytic ion pair monomer comprising 
     (i) the ammonium cation 3-methacrylamidopropyltrimethylammonium and 
     (ii) a sulfonate anion selected from the group consisting of 2-acrylamido-2-methylpropane sulfonate, 2-methacryloyloxyethane sulfonate, vinyl sulfonate, styrene sulfonate and any combination of two or more thereof; 
     (b) at least one comonomer selected from the group consisting of acrylamide, methacrylamide, acrylonitrile, acrylic acid, methacrylic acid, alkali salts of acrylic acid, alkali salts of methacrylic acid, 3-methacryloyloxyethyltrimethylamine, 2-acrylamido-2-methylpropane sulfonic acid, alkali salts of 2-acrylamido-2-methylpropane sulfonic acid, 2-methacryloxyloxyethane sulfonic acid, alkali salts of 2-methacryloyloxyethane sulfonic acid, N-vinyl-2-pyrrolidone and any combination of two or more thereof; and 
     (c) at least one crosslinking agent which has at least two polymerizable olefinic functionalities wherein the olefinic functionalities are suitable for crosslinking. 
     The invention also provides a method of using the polymers of the present invention for absorbing an aqueous electrolyte solution comprising the step of contacting the polymers of the present invention with the aqueous electrolyte solution.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to crosslinked superabsorbent copolymers formedfrom an ampholytic ion pair (3-methacrylamidopropyltrimethylammoniumcation (MPTMA)/sulfonate anion) copolymerized with other comonomers. Afurther aspect of the invention relates to a method of using theaforesaid crosslinked superabsorbent copolymers for absorbing aqueouselectrolyte solutions.

2. Description of the Prior Art

Polymers for absorbing aqueous electrolyte solutions are used innumerous commercial and industrial applications. For example, polymersare used to improve the water absorbency of paper towels and disposablediapers.

Though known water absorbing polymers are highly absorbent to deionizedwater, they are dramatically less absorbent to aqueous electrolytesolutions such as sea water, brine, and urine. For example, hydrolyzedcrosslinked polyacrylamide absorbs 1,024 grams of deionized water pergram of polymer, but only 25 grams of synthetic urine per gram ofpolymer. Crosslinked polyacrylate absorbs 423 grams of deionized waterper gram of polymer, but only 10 grams of synthetic urine per gram ofpolymer. Hydrolyzed crosslinked polyacrylonitrile absorbs 352 grams ofdeionized water per gram of polymer, but only 25 grams of syntheticurine per gram of polymer.

It would be a valuable contribution to the art to develop polymers withhigh absorbency to aqueous electrolyte solutions such as tap water, seawater, brine, and urine. It also would be a valuable contribution to theart to develop inexpensive polymers with high absorbency to aqueouselectrolyte solutions. The market for these types of polymers is largeand the uses are numerous. Therefore, seemingly small improvements inthe absorbency translate into large savings in the quantity of polymerrequired to absorb these aqueous electrolyte solutions and large savingsto the consumer.

SUMMARY OF THE INVENTION

The polymers of the present invention comprise crosslinked polymersformed by the copolymerization of an effective amount of each of thefollowing components to produce a highly absorbent polymer:

(a) an ampholytic ion pair monomer comprising

(i) the ammonium cation 3-methacrylamidopropyltrimethylammonium and

(ii) a sulfonate anion selected from the group consisting of2-acrylamido-2-methylpropane sulfonate, vinyl sulfonate, styrenesulfonate, 2-methacryloyloxyethane sulfonate, and any combinationthereof;

(b) at least one comonomer selected from the group consisting ofacrylamide, methacrylamide, acrylonitrile, acrylic acid, methacrylicacid, alkali salts of acrylic acid, alkali salts of methacrylic acid,3-methacryloyloxyethyltrimethylamine, 2-acrylamido-2-methylpropanesulfonic acid, alkali salts of 2-acrylamido-2-methylpropane sulfonicacid, 2-methacryloyloxyethane sulfonic acid, alkali salts of2-methacryloyloxyethane sulfonic acid, N-vinyl-2-pyrrolidone and anycombination of two or more thereof; and

(c) at least one crosslinking agent which has at least two polymerizableolefinic functionalities wherein each of the olefinic functionalitiesare suitable for crosslinking.

A further aspect of the invention relates to a method of absorbing anaqueous electrolyte solution comprising the step of contacting thepolymers of the present invention with an aqueous electrolyte solution.

Thus, it is a general object of the present invention to provide thepolymers of the present invention having improved absorbency to aqueouselectrolyte solutions.

A further object of the present invention is to provide a method ofusing the polymers of the present invention for absorbing aqueouselectrolyte solutions comprising the step of contacting the polymers ofthe present invention with the aqueous electrolyte solution.

Further objects, features, and advantages of the present invention willbe readily apparent to those skilled in the art upon reading thedescription of the invention which follows.

DESCRIPTION OF THE INVENTION

The present invention provides polymers that are highly absorbent toaqueous electrolyte solutions. Typical aqueous electrolyte solutionsinclude but are not limited to the group consisting of tap water, saltwater, brine, and urine. The polymers of the present invention comprisepolymers formed by the copolymerization of an effective amount of eachof the following components to produce a polymer which is highlyabsorbent to aqueous electrolyte solutions:

(a) an ampholytic ion pair monomer comprising

(i) the ammonium cation 3-methacrylamidopropyltrimethylammonium (alsoreferred to as MPTMA) and

(ii) a sulfonate anion selected from the group consisting of2-acrylamido-2-methylpropane sulfonate (also referred to as AMPS® atrademark of Lubrizol), 2-methacryloyloxyethane sulfonate (also referredto as MES), vinyl sulfonate, styrene sulfonate and any combinationthereof;

(b) at least one comonomer selected from the group consisting ofacrylamide (also referred to as AM), methacrylamide, acrylonitrile (alsoreferred to as AN), acrylic acid (also referred to as AA), methacrylicacid, alkali salts of acrylic acid (also referred to as X-AA), alkalisalts of methacrylic acid, 3-methacryloyloxyethyltrimethylamine,2-acrylamido-2-methylpropane sulfonic acid, alkali salts of2-acrylamido-2-methylpropane sulfonic acid, 2-methacryloyloxyethanesulfonic acid, alkali salts of 2-methacryloyloxyethane sulfonic acid,N-vinyl-2-pyrrolidone and any combination of two or more thereof; and

(c) at least one crosslinking agent which has at least two polymerizableolefinic functionalities wherein each of the olefinic functionalitiesare suitable for crosslinking.

As used in this application, the term "alkali salts" is usedgenerically, unless otherwise indicated, to mean alkali salts includingbut not limited to salts containing lithium, sodium, potassium, andammonium cations.

As used in this application, the term "monomer" is used generically,unless otherwise indicated, to mean monomers, comonomers, termonomers,tetramonomers, etc. The term "comonomer" is used generically, unlessotherwise indicated, to mean monomers, comonomers, termonomers,tetramonomers, etc. for polymers wherein there are at least twodifferent monomers.

As used in this application, the term "polymer" is used generically,unless otherwise indicated, to mean homopolymers, copolymers,terpolymers, tetrapolymers, etc., and thus includes polymers preparedusing any number of monomers. The term "copolymer" is used generically,unless otherwise indicated, to mean polymers prepared using two or moredifferent monomers.

As used in this application, the term "hydrolysis" is used generically,unless otherwise indicated, to include hydrolysis of nitrilefunctionalities and hydrolysis of amide functionalities. Thesehydrolysis reactions are loosely referred to in the art as"saponification." Hydrolysis of these functionalities may occur underacidic or basic conditions. Under basic hydrolysis conditions, the termmay also include, unless otherwise indicated, neutralization ofcarboxylic acid and sulfonic acid functionalities.

The ampholytic ion pair monomer for use in the preparation of thepresent invention may be prepared by methods which are well known tothose skilled in the art. For example, one of the ampholytic ion pairmonomers can be prepared by reacting chloride salt of2-methacryloyloxyethyltrimethylamine in the dark with the silver salt of2-acrylamido-2-methylpropane sulfonic acid or 2-methacryloyloxyethanesulfonic acid at about 20-25 C. to produce the ion-pair monomer inaqueous solution and silver chloride as a precipitate. See J. C.Salamone, L. Quach, A. C. Watterman, S. Krauser and M. U. Mahmud, J.Macromol. Sci.-Chem., A22 (5-7), 653-664.

The olefinic comonomers can include but are not limited to the groupconsisting of acrylamide, methacrylamide, acrylonitrile, acrylic acid,methacrylic acid, alkali salts of acrylic acid, alkali salts ofmethacrylic acid, 3-methacryloyloxyethyltrimethylamine,2-acrylamido-2-methylpropane sulfonic acid, alkali salts of2-acrylamido-2-methylpropane sulfonic acid, 2-methacryloyloxyethanesulfonic acid, alkali salts of 2-methacryloyloxyethane sulfonic acidN-vinyl-2-pyrrolidone and combinations of two or more thereof. All thesesuitable olefinic comonomers are commercially available.

Suitable crosslinking agents can include but are not limited to thegroup consisting of N,N-diallylmethacrylamide, diallylamine,N,N-bisacrylamidoacetic acid, N,N'-bisacrylamidoacetic acid methylester,N,N'-methylenebisacrylamide (methylene-bis-acrylamide),N,N'-benzylidenebisacrylamide, allylacrylate, diisopropenylbenzene,diallyl succinate, ethylene glycol diacrylate, diallylacrylamide,divinylbenzene, and any combination of two or more thereof. All thesesuitable crosslinking agents are believed to be commercially available.

The polymers of the present invention were generally prepared by mixingthe various monomers in the desired stoichiometric ratios in aqueoussolution and then initiating the free-radical copolymerization. Thecopolymerization of the ampholytic ion pair monomer with the olefiniccomonomer and crosslinking agent can be achieved by any of the wellknown free-radical polymerization techniques in solution, suspension, oremulsion environment. Well known azo compounds commonly employed toinitiate free radical polymerization reactions include2,2'-azobis(N,N'-dimethylisobutyramidine) dihydrochloride,azobisisobutyronitrile, 4,4'-azobis(4-cyanovaleric acid),2,2'-azobis(2,4-dimethyl(4-methyoxyvaleronitride),2,2'-azobis(2,4-dimethylvaleronitrile),2,2'-azobis(2-amidinopropane)dihydrochloride,2-t-butylazo-2-cyano-4-methoxy-4-methylpentane, and2-t-butylazo-2-cyano-4-methylpentane, and 4-t-butylazo-4-cyanovalericacid. Well known inorganic peroxide compounds commonly employed toinitiate free radical polymerization reactions include hydrogenperoxide, alkali metal persulfates, alkali metal perborates, alkalimetal perphosphates, and alkali metal percarbonates. Well known organicperoxide compounds commonly employed to initiate free radicalpolymerization reactions include lauryl peroxide,2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane,t-butylperoxypivilate, t-butylperoctoate, p-menthane hydroperoxide, andbenzoylperoxide. The compound t-butylhyponitrite is a well known alkylhyponitrite commonly employed to initiate free radical polymerizationreactions. Furthermore, ultraviolet light is commonly employed toinitiate free radical polymerization reactions. In addition, such othermethods of copolymerization as would have occurred to one skilled in theart may be employed, and the present invention is not limited to theparticular method of preparing the crosslinked polymer set out herein.

These inventive copolymers containing an olefinic comonomer with amide,nitrile, carboxylic acid, or sulfonic acid functionalities orcrosslinking agent with amide, nitrile, carboxylic acid, or sulfonicacid functionalities can optionally be at least partially hydrolyzedand/or neutralized by heating with aqueous base such as aqueous sodiumhydroxide or aqueous potassium hydroxide. The degree of hydrolysisand/or neutralization can be controlled by stoichiometrically limitingthe amount of base relative to the amount of amide, nitrile, carboxylicacid, and sulfonic acid functionalities. If the hydrolysis is carriedout under acidic conditions, the amide and nitrile functionalities canbe converted to carboxylic acid functionalities without neutralizing thecarboxylic acid or sulfonic acid functionalities of the polymer.

The broadest range for the compositions of the inventive crosslinkedMPTMA/sulfonate copolymers is an effective amount of each of theampholytic ion pair monomer, olefinic comonomer, and crosslinking agentto produce a polymer highly absorbent to aqueous electrolyte solutions.

The preferred ranges for the compositions of the inventive polymersgiven in Tables I and II reflect the relative stoichiometric amount inmole percent based on the total number of moles of all the variousmonomers mixed together before the copolymerization. The ratio of thecrosslinking agent to the other monomers is based on the total number ofmoles of the ampholytic ion pair and the comonomers. The actualcomposition of the polymers of the present invention produced by thecopolymerization reaction may vary slightly from the stoichiometricmixture before the copolymerization depending on the reactionconditions.

The broad and preferred ranges for the compositions of the inventivecrosslinked MPTMA/sulfonate copolymers are given in Table I. These broadranges for the compositions of the present invention are based on theexperimental data provided in Example V, Table VII for those polymercompositions which produce an absorbency of at least 70 gram ofsynthetic urine per gram of inventive crosslinked MPTMA/sulfonatecopolymer.

                                      TABLE I                                     __________________________________________________________________________    Broad Compositions For Inventive Polymers                                     MPTMA/sulfonate                                                                              AM  AN  AA  X-AA                                                                              LINK                                           MOLE PERCENT                   *mole ratio                                    __________________________________________________________________________    broad                                                                              15-25     75-85                                                                             --  --  --  0.01-0.3                                       preferred                                                                          20        80  --  --  --  0.01-0.2                                       broad                                                                              40-60     --  40-60                                                                             --  --  0.01-0.3                                       preferred                                                                          50        --  50  --  --  0.01-0.2                                       broad                                                                              40-60     --  --  40-60                                                                             --  0.01-0.6                                       preferred                                                                          50        --  --  50  --  0.01-0.5                                       broad                                                                               3-20     10-15                                                                             --  --  65-87                                                                             0.01-0.3                                       preferred                                                                           3-20     13  --  --  67-84                                                                             0.01-0.2                                       __________________________________________________________________________     MPTMA/sulfonate = 3methacrylamidopropyltrimethylammonium cation/ a            sulfonate anion selected from the group consisting of                         2acrylamido-2-methylpropane sulfonate, 2methacryloyloxyethane sulfonate,      vinyl sulfonate, styrene sulfonate and combinations of two or more            thereof.                                                                      AM = Acrylamide                                                               AN = Acrylonitrile                                                            AA = Acrylic Acid                                                             X-AA = Alkali Salt of Acrylic Acid (Acrylate)                                 LINK = Crosslinking Agent                                                     *mole ratio = mole crosslinking agent per 100 mole of the ampholytic ion      pair monomer and the comonomers.                                         

The more preferred and most preferred ranges for the compositions of theinventive crosslinked MPTMA/AMPS copolymers are given in Table II. Thesemore preferred and most preferred ranges for the compositions of thepresent invention are based on the experimental data provided in ExampleV, Table VII, for those polymer compositions which produce an absorbencyof at least 70 gram of synthetic urine per gram of inventive MPTMA/AMPSpolymer.

                                      TABLE II                                    __________________________________________________________________________    Preferred Compositions For Inventive MPTMA/AMPS Polymers                              MPTMA/AMPS                                                                             AM  AN  AA  X-AA                                                                              LINK                                                 MOLE PERCENT             *mole ratio                                  __________________________________________________________________________    more preferred                                                                        15-25    75-85                                                                             --  --  --  0.01-0.3                                     most preferred                                                                        20       80  --  --  --  0.01-0.2                                     more preferred                                                                        40-60    --  40-60                                                                             --  --  0.01-0.3                                     most preferred                                                                        50       --  50  --  --  0.01-0.2                                     more preferred                                                                        40-60    --  --  40-60                                                                             --  0.01-0.6                                     most preferred                                                                        50       --  --  50  --  0.01-0.5                                     more preferred                                                                         3-20    10-15                                                                             --  --  65-87                                                                             0.01-0.3                                     most preferred                                                                         3-20    13  --  --  67-84                                                                             0.01-0.2                                     __________________________________________________________________________     MPTMA/AMPS = 3methacrylamidopropyltrimethylammonium cation/                   2acrylamido-2-methylpropane sulfonate, 2methacryloyloxyethane sulfonate       AM = Acrylamide                                                               AN = Acrylonitrile                                                            AA = Acrylic Acid                                                             X-AA = Alkali Salt of Acrylic Acid (Acrylate)                                 LINK = Crosslinking Agent                                                     *mole ratio = mole crosslinking agent per 100 mole of the ampholytic ion      pair monomer and the comonomers.                                         

A further aspect of the invention relates to a method of absorbingaqueous electrolyte solutions comprising the step of contacting thepolymers of the present invention with the aqueous solution. Typicalaqueous electrolyte solutions are not limited to but may be selectedfrom the group consisting of tap water, salt water, brine, and urine.For the purpose of this specification tap water is defined to haveelectrolyte concentration of less than 500 ppm of dissolvedelectrolytes, urine is defined to have an electrolyte concentration offrom greater than 500 ppm to at most 10,000 ppm dissolved electolytes,salt water is defined to have an electrolyte concentration from greaterthan 10,000 ppm to at most 34,000 ppm and brine is defined to have anelectrolyte concentation of from greater than 34,000 ppm to thesaturation point of the aqueous solution.

The following examples are intended to illustrate the advantages of thisinvention but are not intended to unduly limit this invention.

EXAMPLE I

The control data in Table III demonstrates that although known polymersare highly absorbent to deionized water, they are dramatically lessabsorbent to aqueous electrolyte solutions such as salt water and urine.Known polymer compositions include crosslinked polyacrylamide, partiallysaponified crosslinked polyacrylamide, crosslinked polyacrylonitrile,partially saponified crosslinked acrylonitrile, crosslinked polyacrylicacid, neutralized crosslinked polyacrylic acid, crosslinked polyacylate,and polymers thereof with sodium 2-acrylamido-2-methylpropane sulfonate.The best of these known polymers absorbs up to about 60 grams of urineper gram of polymer, and most of the known polymers absorb much lessthan 50 grams of urine per gram of polymer.

The polymers of the control data were prepared by mixing the monomers inthe proportions given in Table III in an aqueous solution of deionizedwater. The monomers were present in about 30-40 weight percent relativeto the amount of deionized water. The free radical polymerization wasinitiated with commercially available2,2'-azobis(N,N'-dimethylisobutyramidine) dihydrochloride. About 0.1mole percent based on the total moles of the monomers of the azofree-radical initiator was employed. The reaction mixture was thendegassed by bubbling nitrogen gas through the mixture for 15 minutes.The reaction temperature was maintained between 20°-35° C. for 24 hours.The reactions produced transparent or cloudy hard gels of thecrosslinked polymers. A large volume of deionized water was added to thepolymer product and the polymers were allowed to swell for about 24hours. The swelled polymers were dried in a forced convection oven at74° C. The dried polymers were then mechanically blended to a powder.

Some of the polymers were hydrolyzed and neutralized with a strong basesuch as aqueous sodium hydroxide or aqueous potassium hydroxide. Thedegree of hydrolysis or neutralization could be controlled bystoichiometrically limiting the amount of base relative to the amount ofamide, nitrile, or carboxylic acid functionalities. A suspension of 1gram of the polymer in about 20 milliliters of 0.5 molar aqueous sodiumhydroxide was heated to 95° C. until a light golden-yellow color wasobtained. The mixture was then transferred to a dialysis bag with amolecular weight cut-off of 12,000-14,000 and dialyzed exhaustivelyagainst distilled water until the viscous polymer gel had reached pH 7.This viscous polymer gel was then poured into a plastic dish and driedin a forced convection oven at 74° C. The dried polymers were thenmechanically blended to a powder.

The dried polymers were then tested for deionized water absorption andsynthetic urine absorption. About 1 liter of deionized water orsynthetic urine was added to 0.1 to 0.5 gram of the dried polymer andallowed to stand for 24 hours. The polymer was then separated from theexcess unabsorbed liquid by screening through a 100 mesh per inchstainless steel sieve. The absorbency was determined by weighing theisolated polymer containing the absorbed liquid and substracting theweight of the dry polymer. The absorbency was measured in units of gramsof liquid per grams of polymer. The synthetic urine was prepared bydissolving 0.64 gram CaCl₂, 1.14 gram MgSO₄.7H₂ O, 8.20 gram NaCl, and20.0 gram urea into 1000 gram deionized water. Several of the polymerswere tested two or three times, and the experimental error was withinplus or minus 2-5 percent. This small experimental error was largelycaused by gel blocking and minor diffusion problems that prevented theaqueous liquid from contacting with all the polymer.

                                      TABLE III                                   __________________________________________________________________________    Control Data                                                                  AMPS                                                                              AM AN AA X-AA LINK  XOH  DIW SU                                           MOLE PERCENT      mole ratio*                                                                              g/g**                                            __________________________________________________________________________    --  100                                                                              -- -- --   0.05  NO    17 15                                           --  100                                                                              -- -- --   0.05  YES  1024                                                                              25                                           --  100                                                                              -- -- --   0.05  YES  364 40                                           --  100                                                                              -- -- --   0.20  NO    13 12.5                                         --  100                                                                              -- -- --   0.20  YES  295 16                                           --  -- 100                                                                              -- --   0.05  YES  608 46                                           --  -- 100                                                                              -- --   0.10  NO    0  0                                            --  -- 100                                                                              -- --   0.10  YES  414 42                                           --  -- 100                                                                              -- --   0.20  YES  352 25                                           --  -- -- 100                                                                              --   0.20  NO    21 11                                           --  -- -- 100                                                                              --   0.20  Neutr.                                                                             423 10                                           --  -- -- -- 100(K)                                                                             0.05  NO   669 57                                           --  -- -- -- 100(Na)                                                                            0.05  NO   505 41                                           --  13 -- -- 87   0.05  NO   --  65                                           3   13 -- -- 84   0.05  NO   350 38                                           3   20 -- -- 77   0.05  NO   417 47                                           6   13 -- -- 81   0.05  NO   738 56                                           6   26 -- -- 68   0.05  NO   533 47                                           6   -- -- -- 94   0.05  NO   488 55                                           10  13 -- -- 77   0.05  NO   570 59                                           20  13 -- -- 67   0.05  NO   624 62                                           100 -- -- -- --   0.05  NO   Soluble                                          __________________________________________________________________________     AMPS = 2acrylamido-2-methylpropane sulfonate (Note: AMPS ® is a           trademark of Lubrizol Corporation for 2acrylamido-2-methylpropane sulfoni     acid.)                                                                        AM = Acrylamide                                                               AN = Acrylonitrile                                                            AA = Acrylic Acid                                                             X-AA = Alkali Salt of Acrylic Acid (Acrylate)                                 LINK = Methylenebis-acrylamide Crosslinking Agent                             *mole ratio = mole crosslinking agent per 100 mole of the ampholytic ion      pair monomer and the comonomers.                                              XOH = Basic Hydrolysis and/or Neutralization with aqueous NaOH or KOH         DIW = Deionized Water                                                         SU = Synthetic Urine                                                          **g/g = absorbency units of gram aqueous liquid per gram dried polymer.  

EXAMPLE II

The control data in Table IV demonstrates that although commerciallyavailable water absorbing materials are highly absorbent to water, theyare also dramatically less absorbent to aqueous electrolyte solutionssuch as salt water and urine. The commerically available water absorbingmaterials tested include poly(co-acrylamide-co-acrylic acid) graftedonto starch, a commercial acrylamide polymer sold under the trademark"Water Grabber"® ("Water Grabber" is a trademark of F.P. Products,Inc.), "LUVS"® diaper absorbent ("LUVS" is a trademark of Procter &Gamble Co.), "Pampers"® diaper absorbent ("Pampers" is a trademark ofProcter & Gamble Co.), and "Favor 960"® (Stockhausen, Inc.). The best ofthese known materials absorb up to about 56 grams of urine per gram ofabsorbing material, and most of the known polymers absorb much less than40 grams of urine per gram of absorbing material.

                  TABLE IV                                                        ______________________________________                                        Control Data For Commercial Materials                                                                  DIW     SU                                           Commercial Material      g/g*                                                 ______________________________________                                        COMMERCIAL STARCH-g-POLY(AM-AA)                                                                        345     37                                           WATER GRABBER ® (AM COPOLYMER)                                                                     440     34                                           LUVS ® DIAPER ABSORBENT                                                                            191     16                                           PAMPERS ® DIAPER ABSORBENT                                                                         171     12                                           FAVOR 960 ®          369     56                                           ______________________________________                                         g = graft                                                                     AM = Acrylamide                                                               AA = Acrylic Acid                                                             DIW = Deionized Water                                                         SU = Synthetic Urine                                                          *g/g = absorbency units of gram aqueous liquid per gram dried polymer.   

EXAMPLE III

The homopolymers of the ampholytic ion pair monomers comprising3-methacrylamidopropyltrimethylammonium 2-acrylamido-2-methylpropanesulfonate (MPTMA/AMPS) ("AMPS" is a trademark of Lubrizol Corporationfor 2-acrylamido-2-methylpropane sulfonic acid) or3-methacrylamidopropyltrimethylammonium 2-methacryloyloxyethanesulfonate (MPTMA/MES) with 0.05 weight percent methylene-bis-acrylamidecross linking agent was tested for these absorbency to deionized waterand synthetic urine. The absorbency of homopolymers is very poor. SeeTable V. The absorbency to deionized water is less than 15 gram waterper gram of homopolymer, and only 14 and 24 gram synthetic urine pergram of homopolymer, respectively.

                  TABLE V                                                         ______________________________________                                        Control Data For Ion Pair Homopolymer                                         MPTMA/AMPS    LINK         DIW     SU                                         MOLE PERCENT  mole ratio*  g/g**                                              ______________________________________                                        100           0.05         14      24                                         ______________________________________                                         AMPS = 3methacrylamidopropyltrimethylammonium cation                          2acrylamido-2-methylpropane sulfonate anion                                   MPTAM/MES = 3methacrylamidopropyltrimethylammonium cation                     2methacryloyloxyethane sulfonate anion                                        LINK = Methylenebis-acrylamide Crosslinking Agent                             DIW = Deionized Water                                                         SU = Synthetic Urine                                                          *mole ratio = mole crosslinking agent per 100 mole of the ampholytic ion      pair monomer and the comonomers.                                              **g/g = absorbency units of gram aqueous liquid per gram dried polymer.  

EXAMPLE IV

The control data in Table VI demonstrates that although the knownampholytic ion pair 3-methacrylamidopropyltrimethylammonium2-acrylamido-2-methylpropane sulfonate (MPTMA/AMPS) copolymerized withacrylamide is highly absorbent to deionized water, it is dramaticallyless absorbent to aqueous electrolyte solutions such as salt water,brine and urine. The absorbency to synthetic urine is about the same asfor the better of the known polymers and commercial materials. TheMPTMA/AMPS-acrylamide copolymer also has been grafted onto starch usingceric ion or cobalt-60 irradiation. These starch grafted copolymers arepoorly absorbent to deionized water, and only slightly more absorbent tosynthetic urine. The better of these known polymers absorbs up to about56 grams of urine per gram of polymer, but the rest absorb less than 30grams of urine per gram of polymer.

                                      TABLE VI                                    __________________________________________________________________________    Control Data For Known MPTMA/AMPS-Acrylamide Copolymers                       MPTMA/AMPS   AM Starch                                                                            LINK                                                                              XOH  DIW SU                                           EXP#                                                                              MOLE PERCENT             g/g*                                             __________________________________________________________________________        10       90 --  --  NO   soluble                                          87  10       90 --  0.20                                                                              YES  428 56                                           **  8.56     27.30                                                                            64.86                                                                             --  NO   9.83                                                                              16.21                                        **  8.98     41.76                                                                            49.26                                                                             --  NO   11.54                                                                             16.62                                        **  15.01    64.96                                                                            20.03                                                                             --  NO   14.11                                                                             29.45                                        __________________________________________________________________________     MPTMA/AMPS = 3methacrylamidopropyltrimethylammonium cation/                   2acrylamido-2-methylpropane sulfonate anion                                   AM = Acrylamide                                                               LINK = Methylenebis-acrylamide Crosslinking Agent                             XOH = Basic Hydrolysis and/or Neutralization with aqueous NaOH or KOH         DIW = Deionized Water                                                         SU = Synthetic Urine                                                          *g/g = absorbency units of gram aqueous liquid per gram dried polymer.        **J. C. Salamone, E. L. Rodriguez, K. C. Lin, L. Quach, A.C. Watterson an     I. Ahmed, Polymer 26, 1234-38 (1985).                                    

EXAMPLE V

The polymers of the present invention were prepared according to themethod described in Example I, except that the inventive polymers wereprepared by mixing the monomers in the proportions given in Table VII.

The inventive polymers were tested for absorbency to deionized water andsynthetic urine. The tested polymers of the present invention comprisepolymers formed by the copolymerization an effective amount of each ofthe components as listed in the following tables to produce a polymerhighly absorbent to aqueous electrolyte solutions.

Some of these inventive polymers in this example which contain anolefinic comonomer with amide, nitrile, carboxylic acid, or sulfonicacid funtionalities or crosslinking agent with amide, nitrile,carboxylic acid, or sulfonic acid functionalities were hydrolyzed andneutralized with an aqueous base such a aqueous sodium hydroxide oraqueous potassium hydroxide.

                                      TABLE VII                                   __________________________________________________________________________    Experimental Data For Inventive MPTMA/AMPS Polymers                               MPTMA/AMPS                                                                             AM AN AA X-AA                                                                              LINK  XOH DIW SU                                    EXP#                                                                              MOLE PERCENT          *mole ratio                                                                             g/g**                                     __________________________________________________________________________    87  10       90 -- -- --  0.20  YES 428 56                                    89  20       80 -- -- --  0.02  YES 558 72                                    61  50       50 -- -- --  0.20  YES 271 27                                    66  50       -- 50 -- --  0.20  YES 1099                                                                              93                                    95  20       -- -- 80 --  0.20  YES 428 41                                    19  50       -- -- 50 --  0.50  YES 523 82                                    122 50       -- -- 50 --  0.20  YES 1323                                                                              116                                   221  3       13 -- -- 84  0.05  NO  --  80                                    222  6       13 -- -- 81  0.05  NO  --  93                                    225 10       13 -- -- 77  0.05  NO  --  77                                    223 12       13 -- -- 75  0.05  NO  --  74                                    226 20       13 -- -- 67  0.05  NO  --  70                                    224 24       13 -- -- 63  0.05  NO  --  54                                    __________________________________________________________________________     MPTMA/AMPS = 3methacrylamidopropyltrimethylammonium cation/                   2acrylamido-2-methylpropane sulfonate anion                                   AM = Acrylamide                                                               AN = Acrylonitrile                                                            AA = Acrylic Acid                                                             X-AA = Alkali Salt of Acrylic Acid (Acrylate)                                 LINK = Methylenebis-acrylamide Crosslinking Agent                             XOH = Basic Hydrolysis and/or Neutralization with aqueous NaOH or KOH         DIW = Deionized Water                                                         SU = Synthetic Urine                                                          *mole ratio = mole crosslinking agent per 100 mole of the ampholytic ion      pair monomer and the comonomers.                                              **g/g = absorbency units of gram aqueous liquid per gram dried polymer.  

The data in Table VII demonstrates that these polymers exhibitsignificantly improved absorbency to aqueous electrolyte solutions suchas urine over the absorbency of the polymers listed in Table III, thecommercially available materials listed in Table IV, the crosslinkedMPTMA/sulfonate homopolymers listed in Table V, and the analogouscrosslinked MPTMA/AMPS-acrylamide copolymers listed in Table VI.

The absorbency of these polymers to urine is highly unexpected in viewof the fact that the homopolymers of MPTMA/sulfonate with 0.05 molepercent crosslinking agent only absorb about 24 grams of synthetic urineper gram of the polymer. See Table V. This demonstrates that themonomers when combined into the polymers of the present invention actsynergistically to increase the absorbency of the polymers to aqueousliquids such as salt water and urine.

Taking an absorbency of about 56 grams of synthetic urine per gram ofpolymer as about the best of the known polymers, the preferred polymersof the present invention exceed this absorbency to urine by 25-107percent (70-116 grams synthetic urine per gram of inventive polymer,Table VII, compared to 56 grams urine per gram for the best knownmaterials, Tables III, IV, V, and VI) without sacrificing absorbency todeionized water. These improved absorbencies translate into largesavings in the quantity of polymer required and large savings to theconsumer.

Reasonable variations can be made in view of the foregoing disclosurewithout departing from the spirit or scope of the present invention.

We claim:
 1. A polymer formed by the copolymerization of an effectiveamount of each of the following components to produce a polymer which ishighly absorbent to aqueous electrolyte solutions:(a) an ampholytic ionpair monomer comprising(i) an ammonium cation3-methacrylamidopropyltrimethylammonium and (ii) a sulfonate anionselected from the group consisting of 2-acrylamido-2-methylpropanesulfonate, 2-methacryloyloxyethane sulfonate, vinyl sulfonate, styrenesulfonate and combinations of two or more thereof; (b) at least onecomonomer selected from the group consisting of acrylamide,methacrylamide, acrylonitrile, acrylic acid, methacrylic acid, alkalisalts of acrylic acid, alkali salts of methacrylic acid,N-vinyl-2-pyrrolidone and combinations of two or more thereof; and (c)at least one crosslinking agent which has at least two polymerizableolefinic functionalities wherein each of the olefinic functionalities issuitable for crosslinking.
 2. The polymer of claim 1 wherein thecrosslinking agent is selected from the group consisting ofN,N-diallylmethacrylamide, diallylamine, N,N-bisacrylamidoacetic acid,N,N'-bisacrylamidoacetic acid methylester, N,N'-methylenebisacrylamide(methylene-bis-acrylamide), N,N'-benzylidenebisacrylamide,allylacrylate, diisopropenylbenzene, diallyl succinate, ethylene glycoldiacrylate, diallylacrylamide, divinylbenzene, and combinations of twoor more thereof.
 3. The polymer of claim 2 wherein the crosslinkingagent comprises N-N¹ methylenebisacrylamide.
 4. The polymer of claim 1wherein the polymer is at least partially hydrolyzed.
 5. The polymer ofclaim 1 wherein the polymer is at least partially neutralized.
 6. Thepolymer of claim 1 wherein the comonomer is selected from the groupconsisting of acrylamide, acrylonitrile, acrylic acid, alkali salts ofacrylic acid, and combinations of two or more thereof.
 7. The polymer ofclaim 6 wherein the comonomer is acrylamide.
 8. The polymer of claim 7wherein the polymer comprises a polymer formed by the polymerizationof:(a) in the range of about 15 mole percent to about 25 mole percent ofthe ampholytic ion pair monomer, and (b) in the range of about 75 molepercent to about 85 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.3 mole of the crosslinking agent per 100moles of the ampholytic ion pair monomer and the comonomer.
 9. Thepolymer of claim 8 wherein the polymer is formed by the polymerizationof:(a) about 20 mole percent of the ampholytic ion pair monomer, and (b)about 80 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.2 mole of the crosslinking agent per 100 moles of the ampholyticion pair monomer and the comonomer.
 10. The polymer of claim 7 whereinthe sulfonate anion is 2-acrylamido-2-methylpropane sulfonate.
 11. Thepolymer of claim 10 wherein the polymer is formed by the polymerizationof:(a) in the range of about 15 mole percent to about 25 mole percent ofthe ampholytic ion pair monomer, and (b) in the range of about 75 molepercent to about 85 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.2 mole of the crosslinking agent per 100mole of the ampholytic ion pair monomer and the comonomer.
 12. Thepolymer of claim 11 wherein the polymer is formed by the polymerizationof:(a) about 20 mole percent of the ampholytic ion pair monomer, and (b)about 80 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.2 mole of the crosslinking agent per 100 moles of the ampholyticion pair monomer and the comonomer.
 13. The polymer of claim 6 whereinthe comonomer is acrylonitrile.
 14. The polymer of claim 13 wherein thepolymer comprises a polymer formed by the polymerization of:(a) in therange of about 40 mole percent to about 60 mole percent of theampholytic ion pair monomer, and (b) in the range of about 60 molepercent to about 40 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.3 mole of the crosslinking agent per 100moles of the ampholytic ion pair monomer and the comonomer.
 15. Thepolymer of claim 14 wherein the polymer is formed by the polymerizationof:(a) about 50 mole percent of the ampholytic ion pair monomer, and (b)about 50 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.2 mole of the crosslinking agent per 100 moles of the ampholyticion pair monomer and the comonomer.
 16. The polymer of claim 16 whereinthe sulfonate anion is 2-acrylamido-2-methylpropane sulfonate.
 17. Thepolymer of claim 16 wherein the polymer is formed by the polymerizationof:(a) in the range of about 40 mole percent to about 60 mole percent ofthe ampholytic ion pair monomer, and (b) in the range of about 40 molepercent to about 60 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.3 mole of the crosslinking agent per 100mole of the ampholytic ion pair monomer and the comonomer.
 18. Thepolymer of claim 17 wherein the polymer is formed by the polymerizationof:(a) about 50 mole percent of the ampholytic ion pair monomer, and (b)about 50 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.2 mole of the crosslinking agent per 100 mole of the ampholyticion pair monomer and the comonomer.
 19. The polymer of claim 6 whereinthe comonomer is a alkali salt of acrylic acid.
 20. The polymer of claim19 wherein the polymer comprises a polymer formed by the polymerizationof:(a) in the range of about 40 mole percent to about 60 mole percent ofthe ampholytic ion pair monomer, and (b) in the range of about 40 molepercent to about 60 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.6 mole of the crosslinking agent per 100mole of the ampholytic ion pair monomer and the comonomer.
 21. Thepolymer of claim 20 wherein the polymer is formed by the polymerizationof:(a) about 50 mole percent of the ampholytic ion pair monomer, and (b)about 50 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.5 mole of the crosslinking agent per 100 mole of the ampholyticion pair monomer and the comonomer.
 22. The polymer of claim 19 whereinthe sulfonate anion is 2-acrylamido-2-methylpropane sulfonate.
 23. Thepolymer of claim 22 wherein the polymer is formed by the polymerizationof:(a) in the range of about 40 mole percent to about 60 mole percent ofthe ampholytic ion pair monomer, and (b) in the range of about 40 molepercent to about 60 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.6 mole of the crosslinking agent per 100mole of the ampholytic ion pair monomer and the comonomer.
 24. Thepolymer of claim 23 wherein the polymer is formed by the polymerizationof:(a) about 50 mole percent of the ampholytic ion pair monomer, and (b)about 50 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.5 mole of the crosslinking agent per 100 mole of the ampholyticion pair monomer and the comonomer.
 25. The polymer of claim 6 whereinthe comonomer is a combination of acrylamide and alkali salt of acrylicacid.
 26. The polymer of claim 25 wherein the polymer comprises apolymer formed by the polymerization of:(a) in the range of about 3 molepercent to about 20 mole percent of the ampholytic ion pair monomer, (b)in the range of about 10 mole percent to about 15 mole percent of theacrylamide comonomer, and (c) in the range of about 65 mole percent toabout 87 mole percent of the alkali salt of acrylic acid comonomer,wherein the total amount of the ampholytic ion pair monomer and thecomonomer equals 100 mole percent; and (d) in the range of about 0.01mole of the crosslinking agent to about 0.3 mole of the crosslinkingagent per 100 mole of the ampholytic ion pair monomer and the comonomer.27. The polymer of claim 26 wherein the polymer is formed by thepolymerization of:(a) in the range of about 3 mole percent to about 20mole percent of the ampholytic ion pair monomer, (b) about 13 molepercent of the acrylamide comonomer, and (c) in the range of about 67mole percent to about 84 mole percent of the alkali salt of acrylic acidcomonomer, wherein the total amount of the ampholytic ion pair monomerand the comonomer equals 100 mole percent; and (d) in the range of about0.01 mole of the crosslinking agent to about 0.2 mole of thecrosslinking agent per 100 mole of the ampholytic ion pair monomer andthe comonomer.
 28. The polymer of claim 25 wherein the sulfonate anionis 2-acrylamido-2-methylpropane sulfonate.
 29. The polymer of claim 28wherein the polymer is formed by the polymerization of:(a) in the rangeof about 3 mole percent to about 20 mole percent of the ampholytic ionpair monomer, (b) in the range of about 10 mole percent to about 15 molepercent of the acrylamide comonomer, and (c) in the range of about 65mole percent to about 87 mole percent of the alkali salt of acrylic acidcomonomer, wherein the total amount of the ampholytic ion pair monomerand the comonomer equals 100 mole percent; and (d) in the range of about0.01 mole of the crosslinking agent to about 0.3 mole of thecrosslinking agent per 100 mole of the ampholytic ion pair monomer andthe comonomer.
 30. The polymer of claim 29 wherein the polymer is formedby the polymerization of:(a) in the range of about 3 mole percent toabout 20 mole percent of the ampholytic ion pair monomer, (b) about 13mole percent of the acrylamide comonomer, and (c) in the range of about67 mole percent to about 84 mole percent of the alkali salt of acrylicacid comonomer, wherein the total amount of the ampholytic ion pairmonomer and the comonomer equals 100 mole percent; and (d) in the rangeof about 0.01 mole of the crosslinking agent to about 0.2 mole of thecrosslinking agent per 100 mole of the ampholytic ion pair monomer andthe comonomer.
 31. A method of absorbing aqueous electrolyte solutionscomprising the step of contacting a polymer with an aqueous electrolytesolution wherein the polymer is formed by the copolymerization of aneffective amount of each of the following components to produce a highlyabsorbent polymer:(a) an ampholytic ion pair monomer comprising(i) anammonium cation 3-methacrylamidopropyltrimethylammonium and (ii) asulfonate anion selected from the group consisting of2-acrylamido-2-methylpropane sulfonate, 2-methacryloyloxyethanesulfonate, vinyl sulfonate, styrene sulfonate and combinations of two ormore thereof; (b) at least one comonomer selected from the groupconsisting of acrylamide, methacrylamide, acrylonitrile, acrylic acid,methacrylic acid, alkali salts of acrylic acid, alkali salts ofmethacrylic acid, N-vinyl-2-pyrrolidine and combinations of two or morethereof; and (c) at least one crosslinking agent which has at least twopolymerizable olefinic functionalities wherein each of the olefinicfunctionalities is suitable for crosslinking.
 32. The method of claim 31wherein the crosslinking agent is selected from the group consisting ofN,N-diallylmethacrylamide, diallylamine, N,N-bisacrylamidoacetic acid,N,N'-bisacrylamidoacetic acid methylester, N,N'-methylenebisacrylamide(methylene-bis-acrylamide), N,N'-benzylidenebisacrylamide,allylacrylate, diisopropenylbenzene, diallyl succinate, ethylene glycoldiacrylate, diallylacrlamide, divinylbenzene, and combinations of two ormore thereof.
 33. The method of claim 32 wherein the crosslinking agentcomprises N,N¹ methylenebisacrylamide.
 34. The method of claim 31wherein the polymer is at least partially hydrolyzed.
 35. The method ofclaim 31 wherein the polymer is at least partially neutralized.
 36. Themethod of claim 31 wherein the comonomer is selected from the groupconsisting of acrylamide, acrylonitrile, acrylic acid, alkali salts ofacrylic acid, and combinations of two or more thereof.
 37. The method ofclaim 36 wherein the comonomer is acrylamide.
 38. The method of claim 37wherein the polymer comprises a polymer formed by the polymerizationof:(a) in the range of about 15 mole percent to about 25 mole percent ofthe ampholytic ion pair monomer, and (b) in the range of about 75 molepercent to about 85 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.3 mole of the crosslinking agent per 100moles of the ampholytic ion pair monomer and the comonomer.
 39. Themethod of claim 38 wherein the polymer is formed by the polymerizationof:(a) about 20 mole percent of the ampholytic ion pair monomer, and (b)about 80 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.2 mole of the crosslinking agent per 100 moles of the ampholyticion pair monomer and the comonomer.
 40. The method of claim 37 whereinthe sulfonate anion is 2-acrylamido-2-methylpropane sulfonate.
 41. Themethod of claim 40 wherein the polymer is formed by the polymerizationof:(a) in the range of about 15 mole percent to about 25 mole percent ofthe ampholytic ion pair monomer, and (b) in the range of about 75 molepercent to about 85 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.3 mole of the crosslinking agent per 100moles of the ampholytic ion pair monomer and the comonomer.
 42. Themethod of claim 41 wherein the polymer is formed by the polymerizationof:(a) about 20 mole percent of the ampholytic ion pair monomer, and (b)about 80 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.2 mole of the crosslinking agent per 100 moles of the ampholyticion pair monomer and the comonomer.
 43. The method of claim 36 whereinthe comonomer is acrylonitrile.
 44. The method of claim 43 wherein thepolymer comprises a polymer formed by the polymerization of:(a) in therange of about 40 mole percent to about 60 mole percent of theampholytic ion pair monomer, and (b) in the range of about 40 molepercent to about 60 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.3 mole of the crosslinking agent per 100moles of the ampholytic ion pair monomer and the comonomer.
 45. Themethod of claim 44 wherein the polymer is formed by the polymerizationof:(a) about 50 mole percent of the ampholytic ion pair monomer, and (b)about 50 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.2 mole of the crosslinking agent per 100 moles of the ampholyticion pair monomer and the comonomer.
 46. The method of claim 43 whereinthe sulfonate anion is 2-acrylamido-2-methylpropane sulfonate.
 47. Themethod of claim 46 wherein the polymer is formed by the polymerizationof:(a) in the range of about 40 mole percent to about 60 mole percent ofthe ampholytic ion pair monomer, and (b) in the range of about 40 molepercent to about 60 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.3 mole of the crosslinking agent per 100moles of the ampholytic ion pair monomer and the comonomer.
 48. Themethod of claim 47 wherein the polymer is formed by the polymerizationof:(a) about 50 mole percent of the ampholytic ion pair monomer, and (b)about 50 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.2 mole of the crosslinking agent per 100 moles of the ampholyticion pair monomer and the comonomer.
 49. The method of claim 36 whereinthe comonomer is an alkali salt of acrylic acid.
 50. The method of claim49 wherein the polymer comprises a polymer formed by the polymerizationof:(a) in the range of about 40 mole percent to about 60 mole percent ofthe ampholytic ion pair monomer, and (b) in the range of about 40 molepercent to about 60 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.6 mole of the crosslinking agent per 100moles of the ampholytic ion pair monomer and the comonomer.
 51. Themethod of claim 50 wherein the polymer is formed by the polymerizationof:(a) about 50 mole percent of the ampholytic ion pair monomer, and (b)about 50 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.5 mole of the crosslinking agent per 100 moles of the ampholyticion pair monomer and the comonomer.
 52. The method of claim 49 whereinthe sulfonate anion is 2-acrylamido-2-methylpropane sulfonate.
 53. Themethod of claim 52 wherein the polymer is formed by the polymerizationof:(a) in the range of about 40 mole percent to about 60 mole percent ofthe ampholytic ion pair monomer, and (b) in the range of about 40 molepercent to about 60 mole percent of the comonomer, wherein the totalamount of the ampholytic ion pair monomer and the comonomer equals 100mole percent; and (c) in the range of about 0.01 mole of thecrosslinking agent to about 0.6 mole of the crosslinking agent per 100moles of the ampholytic ion pair monomer and the comonomer.
 54. Themethod of claim 53 wherein the polymer is formed by the polymerizationof:(a) about 50 mole percent of the ampholytic ion pair monomer, and (b)about 50 mole percent of the comonomer, wherein the total amount of theampholytic ion pair monomer and the comonomer equals 100 mole percent;and (c) in the range of about 0.01 mole of the crosslinking agent toabout 0.5 mole of the crosslinking agent per 100 moles of the ampholyticion pair monomer and the comonomer.
 55. The method of claim 31 whereinthe comonomer is a combination of acrylamide and alkali salt of acrylicacid.
 56. The method of claim 55 wherein the polymer comprises a polymerformed by the polymerization of:(a) in the range of about 3 mole percentto about 20 mole percent of the ampholytic ion pair monomer, (b) in therange of about 10 mole percent to about 15 mole percent of theacrylamide monomer, and (c) in the range of about 65 mole percent toabout 87 mole percent of the alkali salt of acrylic acid comonomer,wherein the total amount of the ampholytic ion pair monomer and thecomonomer equals 100 mole percent; and (d) in the range of about 0.01mole of the crosslinking agent to about 0.3 mole of the crosslinkingagent per 100 moles of the ampholytic ion pair monomer and thecomonomer.
 57. The method of claim 55 wherein the polymer is formed bythe polymerization of:(a) in the range of about 3 mole percent to about20 mole percent of the ampholytic ion pair monomer, (b) about 13 molepercent of the acrylamide monomer, and (c) in the range of about 67 molepercent to about 84 mole percent of the alkali salt of acrylic acidcomonomer, wherein the total amount of the ampholytic ion pair monomerand the comonomer equals 100 mole percent; and (d) in the range of about0.01 mole of the crosslinking agent to about 0.2 mole of thecrosslinking agent per 100 moles of the ampholytic ion pair monomer andthe comonomer.
 58. The method of claim 55 wherein the sulfonate anion is2-acrylamido-2-methylpropane sulfonate.
 59. The method of claim 58wherein the polymer is formed by the polymerization of:(a) in the rangeof about 3 mole percent to about 20 mole percent of the ampholytic ionpair monomer, (b) in the range of about 10 mole percent to about 15 molepercent of the acrylamide monomer, and (c) in the range of about 65 molepercent to about 87 mole percent of the alkali salt of acrylic acidcomonomer, wherein the total amount of the ampholytic ion pair monomerand the comonomer equals 100 mole percent; and (d) in the range of about0.01 mole of the crosslinking agent to about 0.3 mole of thecrosslinking agent per 100 moles of the ampholytic ion pair monomer andthe comonomer.
 60. The method of claim 59 wherein the polymer is formedby the polymerization of:(a) in the range of about 3 mole percent toabout 20 mole percent of the ampholytic ion pair monomer, (b) about 13mole percent of the acrylamide monomer, and (c) in the range of about 67mole percent to about 84 mole percent of the alkali salt of acrylic acidcomonomer, wherein the total amount of the ampholytic ion pair monomerand the comonomer equals 100 mole percent; and (d) in the range of about0.01 mole of the crosslinking agent to about 0.2 mole of thecrosslinking agent per 100 moles of the ampholytic ion pair monomer andthe comonomer.
 61. The method of claim 31 wherein the aqueouselectrolyte solution is tap water.
 62. The method of claim 31 whereinthe aqueous electrolyte solution is salt water.
 63. The method of claim31 wherein the aqueous electrolyte solution is brine.
 64. The method ofclaim 31 wherein the aqueous electrolyte solution is urine.